1. Field of the Invention
The present invention relates to a shaft rotation driving apparatus for a machine tool, and more specifically to a shaft rotation driving apparatus for rotating a main spindle or a rotary sleeve fitted to the main spindle, or a shaft rotation driving apparatus for a machine tool including a feed device for moving a movable carriage linearly through a feed screw mechanism or a worm mechanism, etc.
2. Description of the Related Art
As a shaft rotation driving apparatus for a machine tool for rotating a tool or a workpiece to be processed (e.g., for cutting or grinding), there has been so far known such a rear-end motor type spindle rotation driving apparatus that a main spindle rotation driving motor is attached to the rear portion of the main spindle.
In the conventional main spindle rotation driving apparatus of this rear-end motor type, however, since the main spindle rotation driving motor is arranged on the rear side of the main spindle, the axial length of the main spindle apparatus is inevitably increased, with the result that the external shape of the machine tool becomes large when the main spindle apparatus of this type is incorporated in the machine tool. In addition, since the main spindle rotation driving motor is provided at the rear side of the main spindle, there exists a problem in that the rear end side of the main spindle is not open so that it is impossible to assemble a holding device (e.g., a collet chuck for holding a tool) to the main spindle.
Further, a built-in motor type spindle rotation driving apparatus is so far known such that a hollow rotary shaft (sleeve) (to which a rotor is shrinkage fit) is fixed to a main spindle with the use of a key and additionally a wedge ring. On the other hand, a stator is fixed to a main spindle supporting body for rotatably supporting the main spindle.
In the above-mentioned built-in type spindle rotation driving apparatus, although the problem involved in the rear-end motor type can be solved, since the rotor is shrinkage fit to the rotor shaft, an additional processing is required to remove the thermal deformation generated after shrinkage fit processing. Further, when the rotor is provided with a winding coil, since heat is inevitably generated, the temperature of the main spindle rises, thus deteriorating the machining precision by the heat generated by the winding coil. In addition, since the rotor shaft is fixed to the main spindle with the use of the wedge ring, there exists another problem in that the frictional coupling is not stable. Further, in the case where the rotor is a synchronous motor composed of a plurality of combined permanent magnets or a squirrel cage induction motor, there exists another problem in that the maximum rotative speed of the main spindle must be restricted under due consideration of a limitation of breakdown caused by the centrifugal force of the rotating rotor. In addition, in this built-in type motors, since the internal structure is not uniform, there exists other shortcomings that the motor cannot be well balanced in dynamic condition; in particular at high rotative speed. Further, in the spindle rotation driving apparatus of built-in type motor, the composing parts are complicated in shape, and therefore a long assemble and disassemble time is required, thus resulting in a complicated maintenance.
Further, there exists such a quill type spindle rotation driving apparatus that the main spindle itself is fed in the axial direction within a quill by a feed screw shaft rotated by an axial feed motor. In this quill type spindle rotation driving apparatus, on the other hand, the main spindle is rotated by an externally arranged main spindle motor, irrespective of the axial movement of the main spindle. That is, a driven gear rotatably supported by the quill so as not to be moved in the axial direction thereof is fixed to the main spindle via a slide key or splines, and a drive gear driven by the main spindle motor is geared with this driven gear fixed to the main spindle.
In the above-mentioned quill type spindle rotation driving apparatus provided with a feed mechanism, since the main spindle itself is moved in the axial direction thereof by the axial feed motor, it is unnecessary to feed the quill. Further, it is possible to move the main spindle in various ways by combinations of the main spindle feed motion and the quill feed motion. In the spindle rotation driving apparatus of this quill type, however, since the main spindle must be rotated by an external rotating force at any axial feed positions of the main spindle, a key groove long enough to extend over the feed stroke of the main spindle or a sliding transmission mechanism such as spline must be formed in the main spindle, with the result that the mechanical strength of the main spindle is lowered and further the dynamic balancing of the main spindle is deteriorated. In addition, since the main spindle is driven externally, an additional power transmission mechanism such as gear train is required for the motor drive system, thus causing a power loss in the power transmission. Further, since the axial feed motor for rotating the feed screw must be arranged on the rear side of the main spindle, there exists another problem in that the axial length of the main spindle is long and thereby the external shape of the machine tool is large when the main spindle rotation driving apparatus of this type is incorporated in the machine tool.
Further, as a feed device for a machine tool for linearly moving a movable carriage such as a work table, there have been so far known a feed screw mechanism and a worm mechanism.
In the case of the feed screw mechanism, there exists such a type that the feed device is composed of a feed nut member fixed to a movable carriage and a feed screw shaft geared with the feed nut member, and the movable carriage is moved linearly in an axial direction of the feed screw shaft when the feed screw shaft geared with the feed nut member is rotated by an external drive motor connected to the feed screw shaft via a coupling.
In the above-mentioned conventional feed screw mechanism, since the feed screw shaft is connected to and driven by the external drive motor via the coupling, there exists a problems in that the number of parts increases. Further, since the external drive motor is attached to the rear end portion of the feed screw shaft, the size of the mechanism is large. Further, when the feed screw shaft and an external drive motor shaft are not aligned strictly relative to each other, rotative vibration is easily generated and thereby the feed precision of the movable carriage is deteriorated. Further, when the dimension of the feed screw shaft increases, since the inertia thereof increases, it becomes difficult to control the location of the movable carriage at high precision.
Further, there exists another feed screw mechanism such that the feed device is composed of a feed nut member rotatably attached to a movable carriage and a fixed feed screw shaft geared with the feed nut member, and the movable carriage is moved linearly in an axial direction of the feed screw shaft when the feed nut member geared with the feed screw shaft is rotated by a motor mounted on the movable carriage.
In the above-mentioned feed nut mechanism, since the drive motor is arranged on the side of the feed nut member and further the feed nut member must be connected to and driven by the drive motor, the number of parts also inevitably increases. Further, since the drive motor is mounted on the movable carriage, a relatively large space is required for the movable carriage.
Further, in the case of the worm mechanism, the feed device is composed of a worm rack member attached to a movable carriage so as to extend linearly and a worm geared with the worm rack member, and the movable carriage is moved linearly along an extending direction of the worm rack member when the worm geared with the worm rack member is rotated by an external drive motor.
In the conventional worm mechanism, since the external drive motor for driving the worm is additionally mounted separately on one end of the worm along the axial direction thereof, the worm and the motor output shaft must be aligned accurately, and further a relatively large space is required for mounting the external driving motor.